1. Field of the Invention
This invention relates to apparatus for detecting the occurrence of a physiological event based upon an analysis of an electrical waveform associated with the event. The detector may be used as a sense amplifier for use in a pacemaker or it may be used with a cardiac impedance plethysmograph, for use in an automatic implantable defibrillator.
2. Description of the Prior Art
Both demand pacemakers and automatic implantable defibrillators (AID) require circuitry responsive to cardiac activity. In the case of the demand pacemaker, the detection of spontaneous cardiac electrical activity inhibits or prevents the delivery of a stimulating pulse to the heart. In the case of the implantable automatic defibrillator the detection of abnormal cardiac activity is sensed by the AID which delivers a high voltage cardioverting stimulus to the heart. This cardioverting stimulus may injure the patient if it is delivered when it is not needed since it may cause life-threatening arrhythmias.
Consequently, much work has been devoted to developing detectors which reliably sense cardiac activity. In the case of the AID, this is normally accomplished by sensing the mechanical pumping action of the heart as well as the electrical activity generated by the heart. When both the electrical activity and mechanical activity of the heat fail to meet established criteria, the cardioverting output stimulus is produced. Early methods of detecting the mechanical activity of the heart include pressure transducers as taught by U.S. Pat. Nos. 3,614,955 and 3,614,954 to Mirowski et al. These pressure transducers proved unreliable in practice and were replaced by elastomeric transducers as taught by U.S. Pat. No. 3,815,611 to Denniston. Although this form of sensor proved more reliable than the pressure transducer, the difficulty in accurately locating such an elastomeric transducer catheter prevented its widespread adoption.
A further means for detecting cardiac activity comprises a blood impedance plethysmograph which measures the impedance between two spaced electrodes placed on a catheter located within a ventricle of the heart. The volume of blood within the ventricle varies over time as the heart pumps and alters the impedance of the chamber as measured between the spaced electrodes. During normal heart activity the impedance transducer output varies relatively slowly providing a low slew rate signal, which must be detected in the presence of high frequency noise. This problem has not been overcome by prior art detectors such as that taught by U.S. Pat. No. 3,805,795 to Denniston, which do not measure the duration of the impedance signal.